In the finite von Neumann algebra setting, we introduce the concept
of a perturbation determinant associated with a pair of self-adjoint
elements $H_0$ and $H$ in the algebra and relate it to the concept of
the de la Harpe--Skandalis homotopy invariant determinant associated
with piecewise $C^1$-paths of operators joining $H_0$ and $H$. We
obtain an analog of Krein's formula that relates the perturbation
determinant and the spectral shift function and, based on this
relation, we derive subsequently (i) the Birman--Solomyak formula for
a general non-linear perturbation, (ii) a universality of a spectral
averaging, and (iii) a generalization of the
Dixmier--Fuglede--Kadison differentiation formula.